Helicopters are often provided with at least two turboshaft engines. Both engines are connected to the main rotor via a common reduction gearbox, and each of the engines is sized to account for the worst-case scenario of the other engine failing at takeoff. Accordingly, the power of each engine is significantly greater than what is required for cruising.
In cruising conditions, operating a single engine at a relatively high regime instead of both at a lower regime can allow significantly better fuel efficiency. However, once a turboshaft is stopped, there is a significant delay in starting it back up again. This delay is associated with the required amount of time to get the engine running at a sufficient RPM (and draw in a sufficient amount of air) for engine operation to begin. Henceforth, for safety purposes, the typical approach was not to shut down the second engine completely, but to keep it idling, which limited the gain in fuel efficiency.
Accordingly, there remains room for improvement in addressing the fuel consumption of helicopters.